function getData(endpoint) {
  return new Promise((resolve,reject)=>{
     const xhr = new XMLHttpRequest();

     xhr.open('GET', endpoint);
 
    xhr.onreadystatechange = function () {
      if (this.readyState === 4){
        if(this.status === 200){
          resolve(JSON.parse(this.responseText));
        }else{
          reject("出现了问题")
        }
      } 
    };
    
    setTimeout(() => {
      xhr.send(); 
    }, Math.floor(Math.random() * 3000) + 1000);
  
  });


  
}

getData('./movies.json')
 .then(movies => {
 console.log(movies);
 return getData('./actors.json');
 })
 .then(actors => {
 console.log(actors);
 return getData('./directors.json');
 })
 .then(directors324 => {
 console.log(directors);
 })
 .catch(error => {
 console.log("出现了问题");
 });
